Deletions are entire regions of chromosomes that are no longer present in the genome. Chromosomal deletions are powerful tools in the genetic analysis of complex genomes. They have been exploited extensively in several species including Drosophila melanogaster, humans and mice. Drosophila melanogaster is an organism in which deletions in the genome can be efficiently induced and selected. Chromosomal deletions in the genome of Drosophila melanogaster can be induced through the use of either irradiation or treatment with a mutagenic chemical. In humans, deletions that have arisen spontaneously have facilitated the dissection of phenotypes in contiguous gene syndromes. The identification of deletions in the genome of humans has led to the positional cloning of genes with functions critical to the normal functions of the individual. In mice, sets of deletions within particular regions of the genome created by whole animal irradiation experiments have enabled a systematic characterization of functional units along defined chromosomal regions. However, due to the inordinate amount of time required to identify sets of unique deletions within a particular predetermined genetic locus, classical mutagenesis in mice is logistically impractical.
Presently, the most efficient method that can be used to create a chromosomal deletion at a predetermined genetic locus in mice is the Cre/loxP system disclosed in the U.S. Pat. No. 4,959,317. This recombination system uses two DNA constructs that are integrated into predetermined genetic loci on a chromosome, thus enabling the generation of a precise, single deletion in between the two inserted constructs. When a chromosomal deletion is desired in the genome of a mouse, the deletion is first created in an ES (embryonic stem) cell. The ES cell is then injected into a murine multicell blastocyst or morula or aggregated to an earlier stage murine embryo. ES cells used in this manner have the ability of developing into a chimeric mouse which contains the predetermined chromosomal deletion. The Cre/loxP recombination system however, lacks the feature of creating sets of unique deletions at a predetermined genetic locus afforded by classical mutagenesis. In order to create these sets using the Cre/loxP system, it is necessary to produce two new DNA constructs for each desired deletion. Therefore, the Cre/loxP system requires a great deal of effort and time in order to create sets of unique deletions within a predetermined genetic region.